This invention is directed generally to the fastener arts, and more particularly to a sealing threaded fastener having an undercut groove portion beneath a driver head for accommodating a sealing member, to accomplish sealing engagement with a threaded aperture in a workpiece.
Sealing fasteners in general are well-known in the art, and are particularly useful as sealing closures for apertures in metal plates or other surfaces such as in equipment housings or the like. For example, some outboard marine engines include a cast housing unit having a threaded drain opening therethrough and a counter-bore preceding the opening. Heretofore, a threaded fastener having a polypropylene or similar flexible washer or "O-ring" assembled over the thread form was utilized as a drain plug. The polypropylene washer was intended to serve as a seal between the bearing surface of the screw (such as an undersurface of the driver head) and the facing inner-most or bottom-most surface of the counter-bore surrounding the drain opening.
It will be recognized that the flexible washer intermediate the bearing surface of the screw and the counter-bore or other workpiece surface does not permit the bearing surface of the screw to seat positively in metal-to-metal contact with the facing surface of the workpiece. Hence, it is difficult for the installer, and thereafter during oil changes, to ascertain the correct amount of advancement of the plug for proper sealing. If there is too little advancement, the fastener can loosen during operation in response to engine vibration or the like and the seal could be lost due to such loosening. If there is too much advancement, the sealing ring, the threads or both can be damaged, resulting in inadequate sealing.
It is known to provide an undercut groove in the normally flat undersurface of the head to accommodate the sealing member. However, often such sealing members have tended to be extruded out between the screw and workpiece surface, in a process known as extruding "on the take-down face". This results in pinching of the O-ring between the screw and workpiece which can cause damage to the O-ring such that a reliable seal will no longer be maintained.
As an additional matter, conventional cold-forming or roll-forming threading operations generally produce an external screw thread in which one to one-and-one-half pitches of incomplete thread are formed in a "run out portion" adjacent the undersurface of the driver head, due to the inability to position the rolling die sufficiently close to the head and the requirement that the edge of the die exhibit a certain amount of rolling groove draft. More particularly, these incomplete threads present or result in an oversized root diameter which cannot accept the standard female or internal thread form. Accordingly, a conventional roll formed screw fastener cannot normally be advanced sufficiently to obtain metal-to-metal contact between the flat undersurface of the head and a facing workpiece surface. That is, the last one or one-and-one-half pitches of incomplete thread will not be able to enter the internally threaded aperture in a normal fashion. This is because the aperture is provided with complementary internal threads with a crest diameter sized to receive the root diameter of normal, fully-formed threads of the threaded fastener and not the above-mentioned incomplete threads having an over-sized root diameter.
Conventional roll-threaded fasteners are generally formed from a blank which has an initial or unthreaded shank diameter somewhere between the crest diameter and the root diameter of the finished thread to be formed thereon. Hence, the thread is formed essentially by displacing material from portions of the shank to define the thread root portions and moving this displaced material to adjacent portions of the shank to define the thread crest portions. Thus, any incompletely threaded portion of the shank adjacent the undersurface of the head is necessarily of a greater root diameter than the finished thread portion and will be too great in diameter to advance into an internally threaded aperture sized for receiving the finished threads.